Experimental warming and drying increase older carbon contributions to soil respiration in lowland tropical forests

Tropical forests account for over 50% of the global terrestrial carbon sink, but climate change threatens to alter the carbon balance of these ecosystems. We show that warming and drying of tropical forest soils may increase soil carbon vulnerability, by increasing degradation of older carbon. In situ whole-profile heating by 4 °C and 50% throughfall exclusion each increased the average radiocarbon age of soil CO2 efflux by ~2–3 years, but the mechanisms underlying this shift differed. Warming accelerated decomposition of older carbon as increased CO2 emissions depleted newer carbon. Drying suppressed decomposition of newer carbon inputs and decreased soil CO2 emissions, thereby increasing contributions of older carbon to CO2 efflux. These findings imply that both warming and drying, by accelerating the loss of older soil carbon or reducing the incorporation of fresh carbon inputs, will exacerbate soil carbon losses and negatively impact carbon storage in tropical forests under climate change.

Supplementary Fig. 2: Total soil CO2 flux rate at SWELTR partitioned into the root-derived (Root) and soil heterotroph-derived (Soil) components (see Methods).Flux rates are monthly averages for March (dry season) and October (wet season) in 2019 (n = 5 plots).Total flux rates are the same as those shown in Fig. 1a of the main text and are shown here for ease of reference.The figures show plots warmed by +4 o C (WARM) and controls (CTRL).Lines indicate medians, ends of boxes show the upper (Q3) and lower (Q1) quartiles, whiskers indicate minimum and maximum ranges (calculated from quartiles), solid points are individual observations.This partitioning was not affected by experimental warming (p = 0.75) or season (p = 0.35) as tested by two-way ANOVA with treatment and season (Supplementary Table 4).

Dry Wet
Root Soil Total Root Soil Total (for all collar types and seasons) for reference.Bulk soils were collected from all plots in October 2019 from the following depth increments: 0-10, 10-20, 20-50, and 50-100 cm.The figure shows means as large symbols with standard errors and individual measurements as small symbols.n = 5 for bulk soils and for incubations of 0-10 cm depth.n = 3 for incubations below 10 cm depth.Δ 14 C values of CO2 from incubations in the top 20 cm were higher than in situ CO2 (p < 0.01) as tested with a two-way ANOVA with C source and treatment.Effects of experimental warming and differences between Δ 14 C values for bulk soils and CO2 from incubations were tested using a threeway repeated measures ANOVA with C source, treatment.This test indicated a significant interaction between C source and depth (p < 0.01) and comparisons of slopes with depth indicated that bulk soil Δ 14 C values declined more with depth than CO2 from incubations (p < 0.01).Table 6-8).Heterotrophic CO2 fluxes were higher at the drier site (P12) than the wetter site (SL) during the dry-towet transition (p < 0.01, Supplementary Table 7).This partitioning was not affected by throughfall exclusion and did not differ significantly between sites (Supplementary Table 9), but a significant interaction of site and season (p = 0.02) indicated that at P12, more of the total soil CO2 efflux was attributed to heterotrophs during the dry-to-wet season (75 ± 6 %) than in the wet season (41 ± 5 %).This pronounced increase in heterotrophic respiration during the dry-to-wet transition at P12 was likely driven by larger rewetting effects at the drier site, compared to the wetter site.
Supplementary Fig. 6: Δ 14 C values for bulk soil, density fractions CO2 from laboratory soil incubations, and in situ surface soil CO2 efflux at P12 and SL.In situ soil CO2 efflux Δ 14 C values are from this study, are the same as those shown in Fig. 4, and are replotted (for all collar types and seasons) for reference.The figure shows means as symbols with standard errors.n = 3 for bulk soils, incubations, and density fractions from 0-10 cm depth and n = 1 or all sample types at 10-25, and 25-50 cm soil depths.In the 0-10 cm depth, bulk soil and dense fractions have higher Δ 14 C values than respired CO2 (p < 0.01).Cordeiro et al., in review observed that deep roots (90-120 cm) at P12 and SL span a wide range of annual to decadal-aged carbon (Δ 14 C of 4-86 ‰) 2 .The rate of soil respiration was not correlated with its Δ 14 C or δ 13 C signature, although there was a negative correlation between the Δ 14 C and δ 13 C of respired CO2 ( = -0.45,n = 40, p < 0.01).Pearson correlation analysis was used to assess dependence among soil respiration variables and with soil temperature and moisture using the Hmisc (v.5.1.1)package 3 .The δ 13 C of respired CO2 averaged did not differ with experimental drying (p = 0.44) as tested with a three-way repeated measures ANOVA with site, treatment, and season.

Site
for bulk soil, CO2 from laboratory soil incubations, and in situ surface soil CO2 efflux at SWELTR.In situ soil CO2 efflux Δ 14 C values are the same as those shown in Fig.4and are replotted Total soil CO2 flux rate at PARCHED partitioned into the root-derived (Root) and soil heterotroph-derived (Soil) components.Flux rates are single time-point measurements for May (dry-to-wet season transition) and November or December (wet season) in 2019 (n = 5 plots).Total flux rates are the same as those shown in Fig.2aof the main text and are shown here for ease of reference.The figures show plots with 50% of throughfall excluded (DRY) and controls (CTRL).Lines indicate medians, ends of boxes show the upper (Q3) and lower (Q1) quartiles, whiskers indicate minimum and maximum ranges (calculated from quartiles), solid points are individual observations.See Supplementary Fig.5for sites plotted individually and statistical test results.
Total soil CO2 flux rate at P12 (a) and SL (b) partitioned into the root-derived (Root) and soil heterotroph-derived (Soil) components.Flux rates are single time-point measurements for May (dry-towet season transition) and November or December (wet season) in 2019 (n = 5 plots).Total flux rates are the same as those shown in Fig.4of the main text and are shown here for ease of reference.The figures show plots with 50% of throughfall excluded (DRY) and controls (CTRL).Lines indicate medians, ends of boxes show the upper (Q3) and lower (Q1) quartiles, whiskers indicate minimum and maximum ranges (calculated from quartiles), solid points are individual observations.Of total soil respiration, 58 ± 6 % at P12 and 51 ± 8 % at SL was heterotrophic.Effects of throughfall exclusion and season on total, heterotrophic, and root CO2 efflux and the partitioning of total CO2 efflux were tested using a three-way repeated measures ANOVA with site, treatment, and season (Supplementary Single time-point isotopic values of respired CO2 from root exclusion (Root-free) and total soil (Total) respiration collars with and without warming at SWELTR.a. 14 C. b. 13 C. Lines indicate medians, ends of boxes show the upper (Q3) and lower (Q1) quartiles, whiskers indicate minimum and maximum ranges (calculated from quartiles), solid points are individual observations, and n=5.The δ 13 C of respired CO2 averaged -24.7 ± 1.9 ‰ and did not differ with warming (p = 0.85) or season (p = 0.50) as tested with a three-way repeated measures ANOVA with site, treatment, and season.Supplementary Fig. 8: Single time-point isotopic values of respired CO2 from root exclusion (Root-free) and total soil (Total) respiration collars with and without throughfall exclusion.a. 14 C at P12. b. 14 C at SL. c. 13 C at P12. d. 13 C at SL. Lines indicate medians, ends of boxes show the upper (Q3) and lower (Q1) quartiles, whiskers indicate minimum and maximum ranges (calculated from quartiles), solid points are potential outliers, and n=5.